It is known in the art to join together articles of two resins (one colored and opaque and the other colored and transparent) by applying energy with laser beams. This is accomplished by positioning the articles in contact with each other, and transmitting a predetermined amount of laser beam energy focused on the area of contact, thereby causing the junction portion to be melted and joined together (known generally as “laser welding”). Several advantages flow from laser welding. For example, laser welding is widely perceived as a simple, labor saving operation, with attendant improvements in productivity and reductions in production cost. This technique is useful in various applications including the preparation of molded articles suitable for automotive applications.
Recent attention has been directed to laser welding using blends of thermoplastic resins and colorants. The colorants typically contain an organic dye or pigment to control the conversion of laser energy to heat. In a conventional arrangement, the laser beam penetrates through a transparent colored article, positioned closest to laser beam source, and is then largely absorbed in the opaque colored article. The latter article has a relatively higher absorption coefficient in comparison with the transparent article, and this is attributed to the use of an appropriate amount of colorants. The net effect is that the area of contact of the transparent and opaque articles is melted and the surfaces are thereby joined together. See for example Japanese Published (Koukoku) Patent No. 62-49850, Japanese Published (Koukoku) Patent No. 5 (93)-42336.
Other resin composition associated with laser welding are described in U.S. Pat. No. 5,893,959, which discloses transparent and opaque workpiece parts welded together by a laser beam along a joining zone. Both parts contain black dye pigments such as carbon black to cause them to offer a substantially homogenous visual impression even after welding.
However, a difficulty associated with conventional compositions used in laser welding is that when a laser beam is transmitted through a first article (having laser beam transmitting colorants) to a second article (containing laser beam absorbing colorants), the two articles must have different transmission and absorption coefficients. This unfortunately makes it difficult to weld together articles having the same color. Moreover, thermoplastic components may be blackened by the addition of carbon black or nigrosine, as is commonly done in for example automotive applications. However, carbon black and nigrosine cannot transmit a laser beam with a main wavelength (1200 nm to 800 nm), such as Nd:YAG laser and a diode laser.
Surprisingly, it has now been found that including a specific weight percentage of 1:2 type metallic azo complex dye in thermoplastic resin compositions such as polyamide resin compositions used for laser-weldable molded articles results into both the transparent and opaque articles for the laser beam having the same color of black. This allows for a significantly improved transmission coefficient for the near-infrared spectrum of the laser beam, with excellent and balanced heat-resistance and mechanical properties required in automobile application.